1. Field of the Invention
The present invention relates to an inspection apparatus for an electrode plate-connected structure for a secondary cell which is used for optically inspecting a connection between each electrode plate and a corresponding one of two power collecting plates included in an electrode plate-connected structure of a sealed-type secondary cell including a plurality of electrode plates connected to a corresponding one of the two power collecting plates, and the present invention further relates to a method for inspecting the same.
2. Description of the Related Art
A sealed-type secondary battery, such as a nickel-cadmium battery, a nickel metal hydride battery, etc., includes an electrode plate-connected structure including a plurality of positive electrode plates and a plurality of negative electrode plates which are laminated such that two adjacent electrode plates have different polarities and a dielectric separator between them and are connected to a corresponding one of two power collecting plates. The electrode plate-connected structure is sealed together with an electrolyte in a thin battery case having a rectangular solid shape. In the electrode plate-connected structure used for a sealed-type secondary cell, a plurality of rectangular plates having the same polarity are placed in parallel to one another at regular intervals on a rectangular power collecting plate. The plurality of rectangular plates having the same polarity are placed perpendicularly to the rectangular power collecting plate and are integrally connected at a plurality of portions along its side edge to the rectangular power collecting plate by brazing, welding, etc.
FIG. 12 is a front view of a primary part of an electrode plate-connected structure 10 formed in the above-described manner which includes a rectangular power collecting plate 12 and a plurality of electrode plates 11 each similarly having a rectangular shape and longitudinal side edges projecting in the same direction.
The plurality of electrode plates 11 having the same polarity are placed in parallel to one another at prescribed intervals on a surface of the power collecting plate 12 along a longitudinal direction of the power collecting plate 12. A plurality of portions along a side edge of each electrode plate 11 placed on the surface of the power collecting plate 12 are bonded to the power collecting plate 12 by brazing, welding, etc. At the junctions of each electrode plate 11 and the power collecting plate 12, a brazing filler metal coating the surface of the power collecting plate 12 is melted so that fillets 14 which are metallic solidified portions of the brazing filler metal are formed on both sides of each electrode plate 11. Each electrode plate 11 is integrally bonded to the surface of the power collecting plate 12 by the fillets 14.
The fillets 14 are basically formed between adjacent pairs of electrode plates 11 and between the two electrode plates 11 placed furthermost from each other in a width direction of the power collecting plate 12 and a flange of the power collecting plate 12 closest to such two electrode plates 11. The fillets 14 fill the corners made between each of electrode plates 11 and the power collecting plate 12. Each fillet 14 formed between the adjacent pairs of electrode plates 11 has a concave shape and a surface which is curved like the figure U.
In the electrode plate-connected structure 10, uniform formation of a fillet 14 at each corner of an electrode plate 11 and the power collecting plate 12 is not readily achieved. There is a possibility that the fillet 14 is not formed at some corners of the electrode plates 11 and the power collecting plate 12 or a possibility that the fillets 14 are not formed into a shape which can securely bond the electrode plates 11 to the power collecting plate 12.
For example, in an area denoted by A in FIG. 12, the fillet 14 is not substantially present on both sides of the electrode plate 11, such that the bonding strength of the electrode plate 11 with respect to the power collecting plate 12 is not sufficient. In an area denoted by B, the fillet 14 is only present on one side of the electrode plate 11. In such a case, when the bonding strength provided by the fillet 14 formed on the side of the electrode plate 11 is sufficient, no problems are caused.
In an area denoted by C, each fillet 14 formed on both sides of the electrode plate 11 does not have a sufficient thickness in the vicinity of the electrode plate 11, so that this electrode plate 11 cannot obtain sufficient bonding strength from each of the fillets 14.
In an area denoted by D, each fillet 14 formed on both sides of the electrode plate 11 has a thickness gradually reduced in a direction away from the electrode plate 11. However, each of the fillets 14 has a sufficient thickness in the vicinity of the electrode plate 11, so that the bonding strength of the electrode plate 11 with respect to the power collecting plate 12 is sufficient and no problems would be caused.
Thus, each of the fillets 14 in the areas denoted by A, B and C of FIG. 12 does not have a shape which provides sufficient bonding strength to the electrode plate 11 with respect to the power collecting plate 12.
When the fillet 14 has a shape shown in the area denoted by A, B or C, the electrode plate 11 is poorly bonded to the power collecting plate 12, so that there arises a possibility that the electrode plate 11 is readily detached from the power collecting plate 12 through impact, for example. Therefore, before the electrode plate-connected structure 10 including electrode plates 11 connected to the power collecting plate 12 is sealed in a battery case, a bonding state of each of the electrode plates 11 with respect to the power collecting plate 12 is inspected.
In the inspection of a bonding state of each of the electrode plates 11 with respect to the power collecting plate 12, it is determined whether or not a bonding state of each of the electrode plates 11 with respect to the power collecting plate 12 is satisfactory, for example, by charging the power collecting plate 12 of the electrode plate-connected structure 10 with electricity and measuring a drop in voltage at a connection between each of the electrode plate 11 and the power collecting plate 12 based on fluctuations in a resistance value caused by poor bonding.
Japanese Laid-Open Publication No. 8-287962 discloses a method for inspecting a bonding state of battery terminals, though such a method cannot be used for directly inspecting a bonding state of each of the electrode plates 11 with respect to the power collecting plate 12. This method uses an acceleration pickup for detecting oscillation caused by a hammer striking the battery terminals of a storage cell in which an electrode plate-connected structure (corresponding to the electrode plate-connected structure 10) is sealed in a battery case and performs a fast Fourier analysis on the detected oscillation. This method can be applied to determining whether or not a bonding state of the electrode plates and a power collecting plate is satisfactory. Such a determination is performed by striking a battery case in which the electrode plate-connected structure 10 is sealed so as to cause oscillation and analyzing the oscillation.
Japanese Laid-Open Publication No. 2000-100465 discloses a method for detecting whether or not a state of a welded portion of a lead of an electrode plate with respect to an encapsulation structure is satisfactory. Such a determination is performed by detecting a displacement when a tensile force is applied to the welded portion of the lead of the electrode plate and the encapsulation structure and comparing absolute values and differences in the displacement caused at the beginning and end of the application of the tensile force to corresponding prescribed values. This method can be applied to determining whether or not a bonding state of a welded portion of the electrode plates and a power collecting plate is satisfactory. Such a determination is based on an amount of displacement detected by applying tensile force to the welded portion of the electrode plates and the power collecting plate.
However, the following problems arise in the above-described methods.
There is a possibility that the method for determining whether or not a bonding state of the electrode plate 11 and the power collecting plate 12 is satisfactory, based on a drop in voltage at each of the fillets 14, cannot securely detect a poorly-bonded fillet 14, even when one is present. The reason for this is that the electrode plate-connected structure 10 used for a sealed-type secondary cell includes a plurality of electrode plates 11 connected to the power collecting plate 12 by means of a plurality of fillets 14, so that when there are few poorly-bonded fillets 14 and most of the fillets 14 are well-bonded, a drop in voltage over all of the fillets 14 is small.
When the electrode plate-connected structure 10 is sealed in a battery case, a plurality of positive electrode plates 11 and a plurality of negative electrode plates 11 are laminated such that two adjacent electrode plates 11 have different polarities and a separator between them. In such a case, even when the battery case is struck, each of the electrode plates 11 is not sufficiently oscillated. Accordingly, when a bonding state of each of the electrode plates 11 and a corresponding one of two power collecting plates 12 is determined by striking the battery case and analyzing oscillation caused by the impact, there arises a problem that accurate detection of a bonding state of each of the electrode plates 11 and the power collecting plate 12 based on the oscillation is not readily provided.
Moreover, in this case, a bonding state of the plurality of electrode plates 11 and the power collecting plate 12 is determined based on an oscillation state of the entire electrode plate-connected structure 10, so that when the number of the poorly-bonded fillets 14 is small, there arises a possibility that the poorly-bonded fillets 14 are not accurately determined.
In the method for inspecting a bonding state by applying a tensile force to a bonded portion and detecting a subsequent displacement of the bonded portion, each of the electrode plate-connected plates 11 is connected at a plurality of connection portions to the power collecting plate 12, so that there arises a problem that it is not possible to determine whether or not a bonding state of each of the electrode plates 11 is satisfactory as to each of the plurality of connection portions.